Prior art has disclosed and described the application of mechanical oscillators for: measuring fouling deposition; measuring metal loss; and service fluid properties such as density or viscosity. The ability to measure these parameters is linked to the precision and accuracy of measuring the resonance parameters of Q (the quality factor) and the resonance frequency of the mechanical oscillator. The measurement of these resonance parameters may be compromised by the presence of noise. In some cases, the noise may be the inherent measurement reproducibility caused by limitations of the electrical instrumentation. In other cases, noise may be introduced by the environmental effects presented to the mechanical oscillator. These variations are caused by changes in the service environment surrounding the mechanical oscillator. Examples of environmental variables include changes in service fluid density, viscosity, temperature, flow, pressure. For applications directed at measuring service fluid properties (such as viscosity), the prior art identifies algorithms to account for changes in temperature and/or density that occur from a base case calibration. Even for the case where the aforementioned environmental parameters are invariant, fluid flow provides random excitation and relaxation to the mechanical oscillator such as the tines of a tuning fork. These successive excitations and relaxations randomly impact the tines with random phase. This randomness can cause very minor variations in the apparent resonance frequency that would not be observed in the absence of fluid flow. The result is added noise to the measurement of the resonance parameters.
What is absent in the prior art is a methodology to accurately account for the following two separate situations: 1) random variations that occur during the measurement of resonance parameters; and 2) biased drifts in the resonance parameters that smear their determination if the measurement time is sufficiently long to permit a significant drift of the resonance parameters.
The prior art documents the ability to apply signal averaging as a means to reduce variability from noise. Although signal averaging is beneficial and is included in the strategy of this invention, it has negative impact of introducing additional noise. Additional noise is introduced because averaging inherently requires an increase in the time required to collect the data. In cases where the noise is correlated with a process variable (such as a biased increase in temperature), the measurement of the resonance parameters will also be biased. This invention discloses embodiments that enable a reduction of the measurement time to reduce this averaging bias.
The prior art also discloses the possibility of using active excitation frequency sweep methods for measuring the resonance parameters. However, those methods do not consider the presence of noise and can be tedious (time consuming) since they require that the excitation frequency precisely match the resonance frequency. Such methods typically define resonance as the excitation frequency causing maximum oscillator amplitude or minimum excitation current draw. The instant invention does not require that the electrical excitation frequency match the actual resonance of the mechanical oscillator.